As a result of the letter Roosevelt asked Lyman James Briggs, director of the National Bureau of Standards, secretly to organize the Briggs Advisory Committee on Uranium. The committee's first meeting was on October 21, 1939, in Washington, D.C.; $6,000 was budgeted for conducting neutron experiments conducted by Fermi and Szilárd at Columbia.

Four aspects of uranium seem to be critical from the start:

Finding reliable sources of uranium ore in places where the supply cannot be disrupted by other countries.

Developing mass production methods of extracting uranium-235 from ore or creating plutonium.

In England, Otto Frisch and Rudolf Peierls, two researchers at Birmingham University, issued the Frisch–Peierls memorandum in March 1940. The memorandum contradicted the common thinking of the time that many tons of uranium-235 would be needed to make a bomb, requiring delivery by ship. The calculation in the memorandum showed that a bomb might be possible using as little as one pound of uranium-235, and could be quite practical for aircraft to carry.

The MAUD Committee completed the MAUD report on July 15, 1941, and disbanded. The report had two parts: the first concluding that a uranium-235 bomb would be feasible using 26 pounds of active metal with a yield equivalent to 1800 tons of trinitrotoluene (TNT). The second concludes that the controlled fission of uranium-235 could be a source of heat energy for powering machines and a source of radio-isotopes.

On April 14, 1941, Lyman Briggs received a note from Eugene Wigner, stating:

It may interest you that a colleague of mine who arrived from Berlin via Lisbon a few days ago, brought the following message: a reliable colleague who is working at a technical research laboratory asked him to let us know that a large number of German physicists are working intensively on the problem of the uranium bomb under direction of Heisenberg, that Heisenberg himself tries to delay the work as much as possible, fearing catastrophic results of a success. But he cannot help fulfilling the orders given to him, and if the problem can be solved, it will be solved probably in the near future. So he gave the advice to us to hurry up if U.S.A will not come too late.

Marcus Oliphant came to the United States from England in August 1941 to find out why Briggs and his committee were apparently ignoring the MAUD Report. Oliphant discovered to his dismay that the reports and other documents sent directly to Briggs had not been shared with the Advisory Committee. Oliphant then met with the Uranium Committee and his colleagues Ernest Lawrence, James Conant and Enrico Fermi to explain the urgency. In these meetings Oliphant spoke of a "bomb" with certainty and explained that Britain did not have the resources to undertake the project so it was up to the United States.

The next day, the Japanese Empire'sattack on Pearl Harbor led to the United States entry into the war. Four days later, Germany declared war on the United States. At a meeting on December 18 the S-1 project was dedicated to development of a uranium bomb.

As a result of the MAUD Report, the British had started a uranium bomb program referred to by the codenameTube Alloys. Perceived slowness on the part of the United States had become a contentious issue between American and British scientists. Upon entry into the war, the U.S. placed increasing importance on working cooperatively with the British program. Roosevelt wrote a note to Winston Churchill outlining increased U.S.–UK cooperation, but was rebuffed by Churchill. Apparently the British felt the U.S. could add little to the effort at that point. This rebuff turned out to be a major blunder as the U.S. effort quickly caught up with the British effort, and the British realised that their pioneering effort would have no value if it were not quickly capitalized. Leadership of the American atomic (uranium) bomb project was transferred to U.S. ArmyGeneralLeslie Groves from September 1942; Groves (in his own words) had never trusted the British, or anyone else.

On June 17, 1942, Roosevelt approved a proposal by Bush to dissolve the original S-1 Section and created the S-1 Executive Committee, chaired by James B. Conant, with the membership of Briggs, Compton, Urey, Lawrence, and Eger Murphree. The program entered into increased cooperation between the OSRD and the U.S. Army.

As the Army role in the project grew larger, the role of the OSRD became more advisory. Eventually, in May 1943, the Army took full control over the OSRD's research and development contracts, and as such the S-1 Executive Committee became essentially inactive though never formally dissolved. Kenneth Nichols wrote that the committee routinely ratified the MPC's decisions and that The S-1 Committee was eliminated by mid-1943, as it had been superseded by the Military Policy Committee.[1] Bush, Conant, and other OSRD insiders continued their influence in the Manhattan Project through their participation in the Military Policy Committee.